Logic Analyzer probe resistors?

[manlymann]

I need to use a logic analyzer on RS232, modbus etc. Often the bus is more than 5.0VDC.

Is there an inexpensive solution to this? Do I just put a resistor in series with my probe?

[NE666]

First, check the specifications for your particular LA. Many are "5v tolerant", whilst more expensive units often take much more e.g. my MSO 5000 is rated to 40v peak.

If either your unit isn't rated for >5v, or you want to be cautious anyway and protect against any accidental overvoltage, then I suggest using a level-shifter board. They are designed for this, inexpensive and quite plentiful on well-known auction sites.

https://www.pololu.com/product/2595

[egonotto]

I need to use a logic analyzer on RS232, modbus etc. Often the bus is more than 5.0VDC.

Is there an inexpensive solution to this? Do I just put a resistor in series with my probe?

Hello,

why do you think you can be helped with so little information?

Best regards
egonotto

[artag]

Because he didn't know they differed. If he'd known what NE666 has been able to tell him, he probably wouldn't have needed to ask at all. Learning is not just accumulating new facts, it's understanding what it is necessary to know.

[Gyro]

You would be better to use a resistive divide rather than just relying on just a series resistor and the LA's input protection. Depending on the transition speed and LA input capacitance, the value of the resistors could probably be quite high to minimise circuit loading.

[Kean]

Or maybe use proper logic level conversion for the inputs.

There are common circuits and ICs for implementing the physical receiver layer of RS232 (DS1489, MAX3180) and Modbus (RS232,RS485,Ethernet) that provide 3.3V or 5V logic outputs.

[janoc]

Because he didn't know they differed. If he'd known what NE666 has been able to tell him, he probably wouldn't have needed to ask at all. Learning is not just accumulating new facts, it's understanding what it is necessary to know.

But the first step to learning something is to ask questions that can be actually answered.

One doesn't need to be an expert to realize that maybe providing information about which logic analyzer is concerned and what kind of probe one talks about is rather essential information. Saying "my probe" is really not sufficient. People don't read minds.

See also: http://www.catb.org/~esr/faqs/smart-questions.html

[DaJMasta]

Use a scope that can decode it?  If you're not talking about many channels in parallel, just use a scope's analog input channels (with comprehensive protection and probes) and tell the scope to decode digital from those channels.